[Zope] Running different Zope versions

Is it possible to run different Zope versions on one machine? I have a Zope 2.6.4 running and a Zope 2.7.3 both on a Windows 2003 server. They use their own python. I run them on different port numbers configured in the zope.conf file. Both are installed as services. Yet, when both are started usually only the 2.6.4 is available. I have to turn it off to reach the other. What did I forget? Any suggestion appreciated. Henk Laloli